1. Technical Field
The present invention relates to an audio amplifier circuit and more particularly to an improved muting circuit capable of arresting a pop sound and suppressing various noises deriving from a speaker when a power supply of audio equipment is turned on, and making constant the mute time fluctuating in response to the fluctuation of supply voltage, irrespective of the fluctuation thereof.
2. Description of the Prior Art
In audio equipment represented by car stereos, minicomponent stereos, CD radio cassette recorders and the like, the recent trend is for their circuitry to become increasingly integrated, such circuitry including amplifier circuits such as preamplifiers and main amplifiers, and additional circuits such as muting circuits accompanying those preceding ones, and peripheral circuits, as well as AM and FM receiving circuits. Simultaneously with the integration of these various circuits in progress, attempts have also been made to develop the audio amplifier circuit to be integrated in one chip and to digitize it further.
The amplifier circuit fit for use in audio equipment and simultaneously for integration is a direct-coupled amplifier circuit whose power supply circuit supplies, when it is turned on, constant voltage power at various levels to those different circuits stated above. When the power is thus supplied, every circuit is actuated. However, as a transition shock pulse appears at the time the power is supplied, a pop sound is caused to be heard from a speaker. Many people find the pop sound flowing from the speaker to be offensive. This has posed a problem for such audio equipment. Consequently, a muting circuit is operated in conjunction with the on/off timing of the power supply to arrest the sound, the circuit being contrived so as to prevent the pop sound from being produced at the time the power supply is turned on/off.
Incidentally, some audio equipment is often provided with a stand-by switch function as one of its functions, so that while the supply voltage is normally being applied to each circuit, each circuit is caused to operate by supplying power thereto from a power line whenever the stand-by switch is turned on (this is often made an operational power on/off switch). The pop sound is produced from audio equipment of this sort.
FIG. 3(a) shows a conventional muting circuit for placing an audio amplifier circuit in a mute state by utilizing a low-pass filter for feeding a negative feedback signal from the output stage of a main amplifier to the input stage thereof. FIG. 3(b) shows a waveform chart illustrating the operating timing.
In the main amplifier of an audio amplifier circuit reduced to an IC form, a differential amplifier circuit is generally employed at the input stage thereof. FIG. 3(a) shows a differential amplifier circuit 20 as an example of the amplifier circuit of this sort. The differential amplifier circuit 20 comprises PNP bipolar transistors (hereinafter called `transistors`) 21, 22 for differential amplification, and a constant current source 23 located on their upstream side. On the other hand, NPN transistors 24, 25 as current mirror loads connected to the respective collectors of the transistors 21, 22 are provided on their downstream sides.
The base of the transistor 21 receives an input signal via an NPN transistor 26. The emitter of the transistor 26 is connected to a power line +Vcc via a constant current source 27 located on the upstream side of the emitter. The emitter output of the transistor 26 is applied to the base of the transistor 21 and its collector is grounded, whereas its base is connected to an input terminal 28. The input terminal 28 receives the input signal from a preamplifier 30 via a volume control 32 and a capacitor 31, so that the input signal becomes what is applied to the transistor 26.
The output of the differential amplifier circuit 20 is fetched from the collector of the transistor 21 (via a terminal OUT) and fed to an amplifier at the following stage. Moreover, the base of the transistor 22 receives a negative feedback signal (voltage) from the output at the following stage of the main amplifier via a low-pass filter 29. The low-pass filter 29 is an integral circuit having a resistor RNF, a capacitor CNF and a resistor RF.
The muting circuit used in the amplifier circuit of this sort normally utilizes the capacitor CNF of the low-pass filter 29 and combines with a precharge circuit 33 relative to the capacitor CNF. For the precharge circuit 33, a switch circuit is normally employed. With these circuits described above, the main amplifier is placed in a mute state by inhibiting the amplifier from being amplified when the power supply is turned on.
A mute signal is produced by holding on the switch circuit of the precharge circuit 33 for a predetermined period of time in response to the power supply "on" signal. This mute signal causes the capacitor CNF to be charged by receiving current from the power line +Vcc via the precharge circuit 33. FIG. 3(b) shows a change of voltage as the capacitor CNF is charged.
As shown in FIG. 3(b), the capacitor CNF is first precharged by the precharge circuit 33 (switch circuit) that is held "on" at the initial "on" stage of the power supply. Consequently, the voltage rises as the supply voltage rises. The charging voltage simultaneously becomes the base voltage Vb (Vb in the graph of FIG. 3(b)) of the transistors 22. In FIG. 3(b), the constant voltage supplied to the power line +Vcc is represented by Vcc.
The base voltage Vb of the transistor 22 is maintained by precharging at a level higher than the base voltage VIN of the transistor 21 on the input side (voltage VIN is set at a preset operating point). Then the transistor 22 is held in the "off" state, whereby the input signal is cut or attenuated by several tens of dB. The signal amplifying operation of the main amplifier is therefore suspended or inhibited. As a result, the main amplifier is placed in the mute state.
The switch circuit of the precharge circuit 33 is turned off after the capacitor CNF has been precharged for the predetermined period of time. The capacitor CNF then starts to discharge the voltage and therefore the charge is gradually discharged. The discharge voltage is equal to the voltage at the base Vb of the transistor 22. When the discharge voltage becomes substantially equal to the voltage VIN on the input side, the mute state is released, so that the differential amplifier circuit 20 is in an operating condition.
In such a muting circuit, the mute time is determined by the precharge and discharge periods of the capacitor CNF. For this reason, the muting period after the power supply is turned on becomes excessively long if the precharge voltage is set too high. Consequently, one might choose not to set the precharge voltage very high. However, when a pop noise exceeding the precharge voltage is added to the input side, the mute is not effective and this allows the pop sound to flow from the speaker via the main amplifier.
If the precharge voltage is raised to obviate the shortcoming described above, the mute time becomes too long to make difficult the selection of a proper condition. As shown by dotted lines, moreover, the precharge voltage changes as the supply voltage fluctuates. The problem in this case is that the mute time changes.